Generate include/serno.h from the hg revision data.
The code is inspired by the code in atheme-services, but
* it uses 'hg parents' instead of 'hg tip' so it will
also work if the working directory is not updated
to the latest version in the local repository
* the format is different (date-shorthash instead of
revnum:shorthash)
* some sort of serno.h is always generated, be it
"unknown" only